1. Field of the Invention
In general, the invention provides a drier densifier. More specifically, the invention discloses an apparatus and process for drying and densifying plastic material.
2. Prior Art
In the handling and processing of thermoplastic materials the occasion often arises that wet or damp materials need to be dried. Thermoplastic material in a wet or damp state cannot be molded or formed into finished plastic articles. Also, materials are many times in very low bulk density form such as powder or flake and need to be converted to higher bulk density form (i.e., densified). Material in very low bulk density is extremely difficult to process into plastic articles. It is also difficult to convey or transport and expensive to ship due to its low weight per volume.
When thermoplastic materials are produced, the virgin resin is often in a powder form before being extruded into pellets. All plant spillage and waste must be collected, washed, dried and densified to be suitable for use in plastics manufacturing. The processes involved in the manufacturing and finishing of plastic articles frequently produce waste that is fine low bulk density regrind, flake, shavings or dust (e.g., blow molding processes may generate 30 to 50% waste material depending upon the plastic part being formed). Very often this waste must be washed, dried and densified in order to be processed in a secondary operation or sold to a thermoplastic brokerage market as a usable by, product. Furthermore, facilitating reuse of thermoplastic waste material may be beneficial from economic and energy viewpoints since reground waste material can often be extruded at temperatures 10.degree.-20.degree. F. lower than the extrusion temperatures necessary with virgin resin.
A variety of attempts have been made to solve these drying and densification problems. For example, U.S. Pat. No. 4,591,467 to Kopernicky ("Kopernicky") discloses a method for removing moisture and volatiles from molding particulate plastic material feed. Kopernicky teaches the use of flow control means to drop a controlled amount of particulate thermoplastic material into the input end of a plasticizing screw. In order to draw off the hot volatile vapors which are released as the material is gradually melted and compressed by the screw, suction means are operatively associated with the input end of the barrel. The feed-rate of particulate material is restricted so that the barrel of the plasticizing screw is only partially filled near the opening, thereby improving the withdrawal of gases from the barrel. Kopernicky calls for extensive heating (e.g., preheating) and melting (e.g., column 4, lines 13 to 16: "The heat, pressure and shearing forces melt the particles layer by layer gradually releasing the moisture and other volatile as a vapor which flow out of opening 14"). No shaker screen table is disclosed, nor is any reprocessing of non-densified/dried material disclosed.
U.S. Pat. No. 4,970,043 to Doan et al. ("Doan et al.") discloses a process for forming thermoplastic material from granular scrap material. Doan et al. teaches the formation of moldable material from a mixture of granular scrap materials (e.g., reground rubber and a thermoplastic polymer). The material is processed in an extruder under sufficient pressure and temperature (e.g., column 3, lines 49 to 52:500 to 1800 psi and 350.degree. to 500.degree. F.) to form a semi-stable moldable product. The moldable product is subsequently molded. Doan et al. does not provide for reprocessing non-densified/dried material (but rather appears to mandate longer residence times). Finally, Doan et al. does not appear to disclose a partially-filled auger (whereby vapor may escape) since Doan et al. appears to rely upon screw/jacket friction to generate a large part of the high processing temperatures.
U.S. Pat. No. 3.976,730 to Cushing ("Cushing") discloses a method of employing a high percentage of reground thermoplastic resin in an extruder. Cushing teaches mixing virgin polyethylene pellets with low density polyethylene scrap and compressing and melting the mixture within an extruder to form the raw material for newly-molded thermoplastic articles.
While these devices have alleviated many problems, several issues, especially the suboptimization inherently represented by complete melting and imparting of an additional "heat history" with respect to the plastic material have until now not been satisfactorily addressed. The inventive dryer densifier provides a machine uniquely suited to the needs of the plastics industry. This machine will not only densify material of low bulk density, dry material of either high or low bulk density but will accomplish both at the same time and at any mix ratio.